1. Field of the Invention
The present invention relates to an improved multilayer plate for X-ray imaging and to a method for producing such plate, used for converting X-rays into a latent electrostatic image. This latent electrostatic image can subsequently be read out by various schemes, such as by a scanning laser beam, a microcapacitor active matrix panel, or a bank of electrostatic probes.
2. Description of the Prior Art
It is already known to produce multilayer X-ray imaging plates which are sometimes referred to as xeroradiographic plates.
For example, U.S. Pat. No. 3,975,635 of Aug. 17, 1976 discloses a xeroradiographic plate consisting of a conductive backing having thereon a photoconductive layer of selenium and an intermediate layer of an alloy comprising about 15-45 wt % of arsenic and 55-85 wt % of selenium, which intermediate layer has a thickness of about 15-150 .mu.m and is used to reduce the capacitance of the structure with the result that images are obtained which are capable of development at lower fields without substantial loss of resolution.
U.S. Pat. No. 4,286,033 of Aug. 25, 1981 discloses a multilayer inorganic photosensitive device which comprises a number of various layers, one of which is a hole trapping layer consisting of a halogen doped selenium arsenic alloy wherein the amount of selenium ranges from 95-99.9 wt %, the amount of arsenic ranges from 0.1 to 5 wt % and the amount of halogen is from 10-200 ppm (parts per million). This hole trapping layer has a thickness of 0.01-5 .mu.m (microns), and is used to retain positive charges at the interface between the generating layer and the overcoating insulating layer, thereby improving image quality.
U.S. Pat. No. 4,338,387; of Jul. 6, 1982 relates to an overcoated photoreceptive device containing a layer of electron trapping material and a hole trapping layer, these layers being comprised of a halogen doped selenium arsenic alloy wherein the amount of selenium is about 95-99.9 wt %, the amount of arsenic is between 0.1-5 wt % and the amount of halogen is from 10 ppm to 200 ppm.
U.S. Pat. No. 4,770,965 of Sep. 13, 1988 discloses a selenium alloy imaging member suitable for X-ray imaging, which is characterized by providing on the Se alloy layer a thin protective organic overcoating layer having about 0.5-3 wt % of nigrosine. This is claimed to result in a greater resolution at a significantly reduced X-ray dosage. In this U.S. Pat. No. 4,770,965, the concept of using intermediate polymer adhesive primer layers between the selenium layer and the metal oxide surface is also disclosed. However, these polymer layers have high thermal expansion coefficients and are not effective in reducing the shear stress due to different thermal expansion of the various layers in the device and may result in film cracking.
In U.S. Pat. No. 4,891,290 of Jan. 2, 1990 there is disclosed a multilayer photosensitive material for electrophotography, (rather than x-ray imaging) wherein a high surface hardness is obtained by providing a surface protective layer of an arsenic-selenium alloy having a composition of approximately AS.sub.2 Se.sub.3. Such photosensitive material has a high printing resistance. It is also indicated that such photosensitive material may include a buffer layer comprising an arsenic-selenium alloy disposed between the surface protection layer and the charge generation layer which allows for high temperature operation. It should be noted that in electrophotography, to which this U.S. patent relates, the toner particles are mechanically cleaned between images, whereas in digital X-ray imaging there is no mechanical abrasion of the surface and thus a high surface hardness in not required.
In U.S. Pat. No. 4,990,419 of Feb. 5, 1991 assigned to Fuji Electric Co. Ltd., a multilayer electrophotographic photoreceptor is again disclosed, which comprises an As.sub.2 Se.sub.3 carrier transport layer, a 30 to 50 wt % Te-Se alloy carrier generation layer and an As.sub.2 Se.sub.3 surface protection layer as well as an outer layer of a transparent insulating material and in U.S. Pat. No. 5,021,310 of Jun. 4, 1991 also assigned to Fuji Electric Co. Ltd. a further thermal expansion relieving layer comprising arsenic and selenium is provided within the photoreceptor. It is stated in this patent that the As concentration of the thermal expansion relieving layer varied from 10 wt % to 38.7 wt % and its overall thickness was 1 .mu.m. A surface protective layer of As.sub.2 Se.sub.3 containing 1000 ppm of iodine was deposited thereon to a thickness of 3 .mu.m. Again, this patent relates to an electrophotographic photoreceptor, rather than to an X-ray imaging device.
According to U.S. Pat. No. 5,023,661 of Jun. 11, 1991, it has been determined that a fatigue artifact is caused by a defect in the xeroradiographic plate in the form of a selenium crystallite at the lower surface of the selenium layer of the plate, which allows positive charges in the form of holes, to enter the selenium layer from the aluminum base during the transfer step. These are often called "catastrophic spot producing artifacts", and the U.S. patent provides a process for eliminating such artifacts by pre-charging the detector after a thermal relaxation step to eliminate the trapped space charge in the device.
In U.S. Pat. No. 5,320,927 of Jun. 14, 1994 the technology for manufacturing an improved selenium alloy X-ray imaging member on a transparent substrate is examined, wherein a bulk selenium arsenic material containing 0.1 to 0.6 wt % As is evaporated onto said substrate in a controlled fractionation process and the evaporation is discontinued when the weight of the selenium alloy remaining in the boat is 2-10% of the original weight. This patent also teaches the use of a selenium arsenic alloy (1-24% As) between the X-ray absorbing material and the substrate material to reduce the crystallite-induced defects. However, this patent fails to address the issue of mechanical stability of the photoreceptor as well as the space charge neutralization capability of the structure.
In U.S. Pat. No. 5,330,863 of Jul. 19, 1994 a photosensitive material for use in electric photography is disclosed wherein carrier injection preventing layers consisting of selenium/arsenic/sulphur alloy are inserted between the conductive substrate and the carrier transport layer or between the carrier generation layer and the overcoat layer, or between both. This makes the photosensitive material resistant to friction, heat, dark decay and fatigue and exhibits little deterioration under high temperature environments. This patent does not relate to X-ray imaging.
In U.S. Pat. No. 5,396,072 of Mar. 7, 1995 a fairly complex X-ray image detector is disclosed, which comprises a plurality of X-ray sensitive sensors each of which has a collecting electrode, a reference electrode and a switching element which connects the collecting electrode to an output lead; a photoconductor layer is provided between the individual collecting electrodes and a bias electrode; and each of the collecting electrodes comprises two electrically contacting electrode portions arranged and situated in a very specific manner, so that the majority of the charge carriers generated in the photoconductor flow to the collecting electrodes.
In U.S. Pat. No. 5,436,101 of Jul. 25, 1995 an X-ray photoreceptor is disclosed which has a high arsenic interstitial layer 5-40 .mu.m in thickness sandwiched between the substrate and the selenium layer for trapping positive charges injected from the interface. This structure was designed to prevent rather than promote hole injection from the substrate material into the photoreceptor device.
It should be noted that the concept of using multilayer structures based on amorphous selenium alloys (a-Se alloys) originated in the electrophotographic or xerographic industry (see, for example, U.S. Pat. No. 3,041,166 of Jun. 26, 1962) in an effort to make the spectral response of the photoreceptor more panchromatic to compete with the lower cost organic photoreceptors. For example, alloying of Se with about 40 atomic % Te has been shown to decrease the effective optical band gap of selenium from 2.2 eV down to about 1.2 eV. However, this increased longer wavelength photosensitivity generally occurs at the expense of electrophotographic properties--high residual potentials and rapid dark decays being typical of this class of materials. In fact, the electrophotographic properties of a-Se.sub.x Te.sub.1-x, materials, particularly when the Te content is high, generally preclude the use of these materials in monolayer photoreceptor applications. Since photoreceptors require both low residuals, wide panchromicity (especially for laser printer applications, where low cost semiconductor lasers emit light in the long wavelength regime), and low dark decay, considerable effort was placed into decoupling the photogeneration process and the charge transport process in the device. Se.sub.x Te.sub.1-x alloys were used to absorb the light, but since the xerographic properties of this material were not optimal, a second charge transport layer was used to achieve the desired electrophotographic properties.
As is obvious from the various prior art patents referred to above, multilayer selenium based structures have also been employed for higher energy X-ray imaging applications. One of the earliest commercial applications of selenium to X-ray imaging was in xeroradiography, where the detector consisted of a selenium layer deposited onto an aluminum plate. In a typical imaging cycle, the plate was sensitized by corona charging, exposed to the patient modulated X-ray beam to selectively discharge the selenium, and then developed by passing triboelectrically charged toner particles across the selenium plate, transferring the toner particles to paper, and finally fixing the image by heating the paper. Before the next image could be taken, the selenium plate had to be cleaned from all residual toner particles (generally by mechanical brush), and then restored to a "neutral space charge" condition. The multilayer structures used in optical imaging applications and those used in X-ray imaging applications are not interchangeable and have acquired separate status within the relevant art as is obvious from the prior art patents discussed above.
Furthermore, within the X-ray imaging itself there are two distinct modes of imaging, namely the static mode and the dynamic mode which may be defined as follows:
Static Mode Imagine
In the static mode imaging, images can only be taken at a relatively low frequency, e.g. 1 image every 20 seconds, and the X-ray beam is pulsed. As such, there is sufficient time to neutralize any space charge which accumulates in the device between images.
Dynamic Mode Imaging
In the dynamic mode imaging, images are taken at a much higher frequency, e.g. 30 images per second, and the X-ray beam is left on during the entire examination. In this case, there is no time to remove the applied bias voltage between images to allow holes to be injected from the bottom buffer layer into the bulk X-ray absorbing layer to neutralize the negative space charge.
Although the above discussed prior art indicates that a considerable amount of work is being done in the area of optical and X-ray imaging technologies, until now, selenium based X-ray detectors have suffered from the presence of polycrystallites in the selenium layer located near the substrate. The presence of such polycrystallites is undesirable in X-ray imaging applications, since it could lead to spurious charge injection sites and in the extreme case to a loss of the imaging capabilities for X-ray imaging detectors where the latent electrostatic image is read from the substrate. The manufacturing process window for producing a layer which is free of polycrystallites at the interface while simultaneously keeping the bulk properties of the amorphous selenium layer at their optimal value is extremely narrow.
Furthermore, until now selenium-based X-ray detectors have suffered from thermal shocks which often lead to the physical delamination of the selenium film from the substrate due to the stress resulting from the mismatched thermal expansion between the bulk amorphous selenium layer and typical substrate materials such as glass and aluminum.
Moreover, prior art selenium based X-ray detectors have suffered from the availability of a limited number of materials which could be used as the substrate electrode material. For example, aluminum has been widely used because of its high oxidation potential and hence its ability to form a high-quality uniform aluminum oxide layer to prevent electron injection from the substrate into the bulk of selenium. Another example is Indium Tin Oxide (ITO) coated glass which has shown some electron blocking characteristics at the ITO selenium heterojunction.
However, known detectors do not normally allow the use of a wide variety of substrate materials because they rely on the electrochemical interaction between the materials to create the required electron blocking characteristic.
In addition, prior art selenium-based X-ray detectors have suffered from memory effects induced by the accumulation of negative space charge in the doped selenium layer. Laborious erasing schemes utilizing a combination of light, temperature and voltage polarization cycles were necessary to erase the accumulated space charge. In the case of opaque substrate materials, this prohibits the use of light in the erasure sequence.
Finally, known selenium-based X-ray detectors have suffered from difficulties in applying the high voltage bias across the doped amorphous selenium layer. This problem was handled by either corona charging the device or by inserting insulating materials such as polycarbonate, polyester, parylene or glass between an upper electrode and the doped amorphous selenium layer to prevent spurious hole injection from the electrode into the selenium layer. None of these approaches allow for imaging at fluoroscopic rates (30 images/second).